This invention relates to a method for correcting the "Narcissus Effect" in a scanning infrared microscope employing a cooled infrared detector.
Scanning infrared radiometers have found important applications in non-destructive testing of miniature and microelectronic components, chips and printed circuits. Characteristic thermal patterns for a component or circuit can be determined and measured by scanning a cooled infrared detector over the target area. Magnifying objective lenses are employed for imaging individual pixels of the field of view on the detector as it is scanned over the target. Cooling of the detectors to extremely low temperatures is required for sensivity.
It has been found in scanning radiometers of the type described that low emissivity targets have provided seriously erroneous temperature readings even when the emissivity of the target has been measured and is used in computing the temperature of the various components or pixels of the target. It has been found that the errors are caused by viewing the cold detector and its associated Dewar through reflections from the target or specimen which is known as the "Narcissus Effect". If the emissivity of the target, which is defined as the ratio of the radiation emitted by an object to the radiation that would be emitted by a black body at the same temperature, is unity or that of the black body, there would be no problem. However, as a practical matter the emissivity of all materials is less than one, and the lower the emissivity of the target the worse the "Narcissus Effect".